Chickenpox is caused by varicella-zoster virus (VZV), a member of the herpesvirus family. The disease occurs in people with no prior immunity to VZV. VZV-specific antibodies can be demonstrated shortly after the onset of disease, decline during convalescence, but remain detectable for many years and correlate with immunity to the disease. Chickenpox is highly contagious; over 90% of the population becomes exposed to VZV before the age of 20. In most or all cases, VZV becomes latent, possibly in dorsal root ganglion cells. From this latent state, VZV can reactivate and cause zoster even in the presence of specific antibodies, probably as a result of weakened cellular immunity. The disease is highly morbid to the immunosuppressed and to those beyond the second decade.
In 1974, Takahashi reported the isolation of the Oka strain of VZV from the vesicle of a child with chickenpox. This strain then was attenuated by passage through guinea pig embryo cells and human diploid fibroblasts. The attenuated variant of VZV/Oka has been tested clinically in thousands of youngsters. It is capable of eliciting high levels of antibodies reactive with the surface of the VZ virion. Furthermore, this strain displays protective efficacy for the prevention of chickenpox in young children and in the immune-compromised. It is noteworthy that this strain of VZV is the only available viral vaccine which can be used safely in immune-compromised patients, thus making VZV versatile for broader applications.
Epstein-Barr virus (EBV) is the etiologic agent of infectious mononucleosis. The EB virion has 3 major surface glycoproteins: gp350 (350,000 dalton glycoprotein), gp220, and gp85. The gp350 and gp220 polypeptides are the products of a single viral gene. These 2 glycoproteins are capable of eliciting the production of antibodies capable of neutralizing EBV infectivity in vitro. Therefore, the gp350 gene and its products are useful for the preparation of a vaccine to EBV-induced disease through the use of recombinant DNA techniques. Furthermore, it would be desirable to vaccinate people simultaneously against both VZV- and EBV-induced diseases, or alternatively against both VZV-induced disease and another disease.
Hepatitis B virus (HBV) is the infectious agent responsible for several varieties of human liver disease, including cirrhosis and hepatocellular carcinoma, which claims hundreds of thousands of lives per year. The HB virion is composed of two groups of structural proteins, the core proteins and the envelope or surface ("S") proteins. In addition to being the major surface proteins of the virion, i.e., Dane particle, the "S" proteins are the sole constituents of Australia antigen, or 22 nm particles. The "S" proteins are the translational products of a large open reading frame (ORF) encoding 389-400 amino acids, depending upon serotype. This ORF is demarcated into three domains, each of which begins with an ATG codon that is capable of functioning as a translational initiation site in vivo. These domains are referred to as preS-1 (108-119 amino acids), preS-2 (55 amino acids), and S (226 amino acids) in their respective 5'-3' order in the gene. The six protein products derived from this ORF have the following compositions:
1) gp42 (42,000 dalton glycoprotein)=preS-1/preS-2/S (meaning preS-1, contiguous with preS-2, contiguous with S) PA1 2) p39 (p=protein)=preS-1/preS-2/S PA1 3) gp36=preS-2/S (two glycosylation sites) PA1 4) gp33=preS-2/S (one glycosylation site) PA1 5) gp27=S (one glycosylation site) PA1 6) p24=S (or HB surface antigen, i.e., HBsAg)
Outside of man, chimpanzees are the only species which is fully susceptible to HBV infection, as reflected in quantifiable markers such as HBs.sup.+, elevated serum levels of liver enzymes, etc. Chimpanzees have been vaccinated with three doses of purified HBsAg particles (containing p24) and then challenged with a large dose of infectious HBV. While mock-vaccinated animals have suffered the signs of acute HBV infection, the HBsAg-vaccinated animals have been protected completely from any signs of infection. Therefore, in this experimental system, HBsAg particles, composed of gp27 and p24 (S domain only), have been sufficient to induce protective immunity. Spurred by these observations, several manufacturers have produced HB vaccines composed of HBsAg particles.
Recent data have suggested that the preS-1 and preS-2 domains may play an important role in immunity to HBV infections. Both antibodies to preS-1 (elicited by immunization with a peptide consisting of amino acid residues 10-32 of preS-1) as well as antibodies to preS-2 (elicited by immunization with a peptide consisting of amino acid residues 1-26 of preS-2) are capable of blocking the binding of HBV to human hepatoma cells in vitro; anti-HBs (sera from patients vaccinated with HBsAg lacking preS-1 or preS-2) is incapable of mediating this blocking event. If this in vitro event mimics in vivo infection, then pre-S (i.e., preS-1 and preS-2 in toto linked together) domains may represent the HBV binding site to its liver cell receptor, and anti-pre-S may block HBV attachment and initiation of infection. In addition, it has been found that anti-pre-S rises in titer during the recovery phase from acute HBV infection, indicating a role for these antibodies in recovery. Finally, it has been shown that vaccination of chimpanzees with a 108 amino acid pre-S polypeptide (residues 27-119 of preS-1 contiguous with 1-16 of preS-2) was capable of mediating some measure of protection against HBV challenge. In sum, these experimental observations have suggested that the pre-S domains are a useful addition to present HB vaccines, thus highlighting the desirability of expressing the large ORF encoding preS-1/preS-2/S.